**From:** Doug McKean (*dmckean@corp.auspex.com*)

**Date:** Thu May 25 2000 - 16:49:10 PDT

**Next message:**Steeve Gaudreault: "[SI-LIST] : 5V PECL vs 3.3V PECL - VTT???"**Previous message:**Doug McKean: "Re: [SI-LIST] : RE: Split Plane"**In reply to:**Doug McKean: "Re: [SI-LIST] : RE: Split Plane"**Next in thread:**Barry Ma: "[SI-LIST] : RE: Split Plane"

Woops ...

Didn't mean to start anything.

The following derivation uses the -3dB point

or critical frequency for a passive circuit.

- Doug McKean

--------------------------------------------------

Assume V(0) = 0 and V(infinity) = Vmax

Look at only V = 10%Vmax and V = 90%Vmax

Since, V = Vmax*[1 - exp(-t/tau)]

(tau being whatever time constant you wish)

----------

V at 10% ...

V(0.1) = 0.1*Vmax = Vmax - Vmax*exp(-t/tau)

Vmax*exp(-t/tau) = Vmax - 0.1*Vmax

= 0.9*Vmax

exp(-t/tau) = 0.9

ln[exp(-t/tau)] = ln[0.9]

(-t/tau) = -.1

t(10%) = 0.1*tau

----------

V at 90% ...

V(0.9) = 0.9*Vmax = Vmax - Vmax*exp(-t/tau)

Vmax*exp(-t/tau) = Vmax - 0.9*Vmax

= 0.1*Vmax

exp(-t/tau) = 0.1

ln[exp(-t/tau)] = ln[0.1]

(-t/tau) = -2.3

t(90%) = 2.3*tau

----------

Trise = t(90%) - t(10%)

= 2.3*tau - 0.1*tau

= 2.2*tau

Since, f = 1/(2*pi*tau)

makes tau = 1/(2*pi*f)

Trise = 2.2*tau

= 2.2/(2*pi*f)

= 0.35014/f

f = 0.35014/Trise

----------

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**Next message:**Steeve Gaudreault: "[SI-LIST] : 5V PECL vs 3.3V PECL - VTT???"**Previous message:**Doug McKean: "Re: [SI-LIST] : RE: Split Plane"**In reply to:**Doug McKean: "Re: [SI-LIST] : RE: Split Plane"**Next in thread:**Barry Ma: "[SI-LIST] : RE: Split Plane"

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